Test strips are often used to measure the presence and/or concentrations of selected analytes in test samples. For example, a variety of test strips are used to measure glucose concentrations in blood to monitor the blood sugar level of people with diabetes. These test strips include a reaction chamber into which a reagent composition has been deposited. Current trends in test strips require smaller test samples and faster analysis times. This provides a significant benefit to the patient, allowing the use of smaller blood samples that can be obtained from less sensitive areas of the body. Additionally, faster test times and more accurate results enable patients to better control their blood sugar level.
In connection with smaller sample volumes, it is known to provide test strips having a sufficiently small reaction chamber such that sample fluid is drawn therein by capillary action, which is a phenomenon resulting from the surface tension of the sample fluid and the thermodynamic tendency of a liquid to minimize its surface area. For example, U.S. Pat. No. 5,141,868 discloses a test strip having a cavity sized sufficiently small to draw sample liquid therein by capillary action. The cavity is defined by two parallel plates spaced about 1 mm apart by two epoxy strips extending lengthwise along lateral sides of the plates. The cavity is open at both ends, one of which receives the sample, and the other of which allows air to escape. The cavity includes an electrode structure and carries a coating of a material appropriate to the test to be performed by the test strip.
Numerous variations of the reagent coating are possible depending upon the specific analyte(s) to be tested, and there are typically numerous chemistries available for use with each of the various analytes. Generally speaking, however, it is desirable to form the reagent layer in the test strip or biosensor as thin and as uniform as possible. For example, as sample volumes and thus the size of the sample-receiving chamber continue to get smaller, variations in thickness of the reagent layer increasingly affect the accuracy of the test result. Further, in test strips having smaller cavities, the reagent layer must be thinner in order to leave ample space in the chamber to receive the sample. Additionally, a thinner layer will hydrate more quickly and will therefore produce a quicker test result.
While forming a thin and uniform reagent layer that hydrates quickly in an extremely small volume is desirable, it is not easily obtained because of the difficulties in working with very small volumes of liquid reagent. For example, one prior art approach to forming the reagent layer is to deposit the same into the sample-receiving chamber after the latter is formed in the test strip. However, this can result in a more uneven reagent layer due to phenomena such as the formation of a meniscus at the perimeter of the chamber, which in turn results in the reagent having a different thickness adjacent to the side walls of the chamber than in the interior portion. This can cause inconsistency in the filling of the chamber, prolonged dissolution intervals, and inconsistent mixing of the reagent with the sample fluid, and, ultimately, poor test results.
It is known to provide elongated webs of test strip material on which a continuous stripe of reagent is deposited. The test strips are then cut from the elongated web, the cuts extending through the reagent layer. Typically in such test strips so formed, the sample-receiving chamber has a floor with two different levels, a lower level provided by a base substrate material, and an elevated level provided by the reagent layer. The discontinuities in the floor of the chamber can be problematic. First, since the top of the reagent layer must be spaced typically a minimum distance from the ceiling or top of the chamber to allow ample space for sample, the lower height of the base substrate undesirably creates wasted chamber space.
Second, the edges of the reagent layer that are formed by the reagent stripe will likely be uneven in thickness and width. For example, the width of the reagent stripe can vary substantially as it is applied to a long web of material, which in turn results in some strips having more reagent than others after the strips are cut from the web. Further, the edge quality of the reagent stripe is highly variable, depending upon many factors, such as the means used to apply the stripe, its viscosity and the like. These inhomogeneities can lead to inaccuracies in determining analyte concentration.
It is an object of the present invention to form a test strip having a small sample-receiving chamber but including a reagent layer therein that avoids the drawbacks discussed above.